Micro electro-mechanical systems (MEMS) motion sensors are now widely used in automotive electronics, medical equipment, hard disk drives, and portable consumer electronics. For example, a typical smart phone includes a MEMS accelerometer, gyroscope and possibly a compass. Single-axis MEMS accelerometers detect a change in velocity in a given direction, which are almost universally used to inflate automotive airbags in the event of crashes. They are also used as vibration sensors to detect bearing wear in machinery, since vibration can be modeled as acceleration and deceleration happening quickly in a periodic manner. Two-axis accelerometers implement a second dimension, which can be as simple as detecting tilt by measuring the effect of gravity on the X-Y axis of the accelerometer. Three-axis accelerometers can detect motion in three different directions, which are widely used in mobile devices to incorporate tap, shake and orientation detection. Gyroscopes are basically three-axis inertial sensors. Multi-axis MEMS gyroscopes are often embedded along with three-axis accelerometers in inertial measurement units.
A capacitor is one of key components in determining MEMS device performance. The square measure of a capacitor contributes to the sensitivity of motion sensing circuit. The movable MEMS components with relatively larger dimension can get easily stuck, either to another movable MEMS component or to MEMS structures surrounding it.
Stiction is considered a significant problem in MEMS motion sensors. Internal MEMS structures are so small that surface forces can cause microscopic structures to stick together when their surfaces come into contact. Existing methods can only overcome the stiction issue by losing design performance, e.g. by compromising sensitivity performance of the MEMS motion sensor, and are thus not entirely satisfactory.